The inventive concept relates generally to nonvolatile memory devices. More particularly, the inventive concept relates to operating methods and data read methods in nonvolatile memory devices.
The individual memory cells in a nonvolatile memory device, such as a flash memory device, store data in accordance with a plurality of threshold voltage distributions, where each respective threshold voltage distribution is assigned a corresponding logic state for stored data. The data stored by a memory cell may then be read by determining whether the memory cell is turned ON/OFF when a predetermined read voltage is applied.
During (and/or following) the programming of a memory cell, its intended threshold voltage distribution may be undesirably shifted or broadened due to a number of events or conditions including (e.g.,) charge leakage, program disturbances, word and/or bit line coupling, temperature change, voltage change, etc. Once the threshold voltage distribution of a memory cell has been thus altered, it become difficult accurately read the logic state of the stored data. In some instances, the logic state becomes indiscernible and a read fail occurs. Once a read fail occurs, conventional nonvolatile memory devices may perform a so-called “read retry operation”. One example of a read retry operation is disclosed, for example, in published U.S. Patent Application No. 2010/0322007, the subject matter of which is hereby incorporated by reference.
In general during the read retry, a nonvolatile memory device may iteratively perform a read operation. During each successive iteration of the read operation the level of the applied read voltage is sequentially increased or decreased until the read operation does not result in a read fail. However, the use of repeated read operations during a read retry operation invariably extends the time required to successfully read data from the nonvolatile memory device.